Generally, solar cells for photovoltaics can be made of monocrystalline silicon or polycrystalline silicon. Whereas high-quality solar cells are made of silicon monocrystals, which is technologically more complex and thus more costly, less expensive solar cells are conventionally made of polycrystalline silicon, which is less complex and thus more cost-effective. Specifically in the production of polycrystalline silicon, features leading to a reduction in costs and in technological complexity therefore play an important part.
Conventionally, the melting crucible is filled with lumpy silicon. During the subsequent melting-on to form liquid silicon, there occurs in this case considerable volumetric shrinkage, caused by the significantly differing densities of molten silicon as compared to the previously present feedstock. Thus, in the case of conventional methods, only a small portion of the melting crucible volume can be effectively used. Various measures are known from the prior art to compensate for the volumetric shrinkage.
U.S. Pat. No. 6,743,293 B2 discloses a method for producing polycrystalline silicon, wherein an annular attachment having a corresponding profile is attached to the upper edge of the melting crucible in order to form overall a container construction having a larger volume. A silicon feedstock is introduced into the container construction. After the melting of the silicon, the silicon melt fills out the entire melting crucible, but not the volume enclosed by the annular attachment. However, the container construction requires a crystallization system having a greater volume, in particular a greater height; this is undesirable for reasons relating to energy. Furthermore, it is difficult to provide a suitably dimensionally stable annular attachment for reuse.
As an alternative to the above method, in crystallization systems which operate using the Czochralski method, continuous or discontinuous replenishment of lumpy raw material is known in order at least partly to compensate for the volumetric shrinkage caused by the melting of the raw material in the melting crucible.
EP 0 315 156 B1, which corresponds to U.S. Pat. No. 5,080,873, discloses a crystallization system of this type in which crystalline material is supplied to the melting crucible via a supply pipe. Deceleration means in the form of cross-sectional constrictions or profile bends are provided in the supply pipe in order to reduce the falling speed of the crystalline material. Active preheating of the crystalline material is not disclosed.
EP 1 338 682 A2, which corresponds to US 200310159647A1, discloses a crystallization system using the Czochralski method, wherein crystalline material slides into the melting crucible via an inclined pipe. JP 01-148780 A and English Abstract disclose a corresponding construction. However, in this case, complex measures must be taken to allow the introduction of crystalline raw material into the melting crucible without splashing. The reason for this is that splashing of the hot melt in the crystallization system leads to damage of components and to impurities which can be removed again only with difficulty. Active preheating of the crystalline material is not disclosed.
US 2004/0226504 A1 discloses a complex flap mechanism for suitably reducing the falling speed of the crystalline material during pouring into the melting crucible. US 2006/0060133 A1 discloses a crystallization system in which crystalline silicon falls from a vertical pipe down into the melting crucible. The lower end of the pipe is sealed by a conical shut-off body which imparts a radial movement component to the crystalline material. Active preheating of the crystalline material is not disclosed.
An alternative to the aforementioned mechanical solutions is a suitable selection of the process parameters in order partly to solidify the surface of the melt at the point in time at which crystalline material is replenished. This is disclosed for example in JP 11/236290 A or JP 62/260791 A and English Abstract thereof. However, the solidification of the surface of the melt in the melting crucible leads to undesirable slowing-down of the process.
EP 1 337 697 B1, which corresponds to U.S. Pat. No. 6,454,851 B1, discloses a crystallization system using the Czochralski method, wherein crystalline silicon is deposited only on islands of still solid silicon. These islands have to be determined with the aid of a video system and complex image evaluation. In order to strike these islands, the conveying means for conveying the crystalline silicon has to be moved in a suitable manner into the melting crucible, and this is complex.
In all of the crystallization systems operating using the Czochralski method, the melting crucible is heated from the base. In the production of crystalline materials using the VGF method, the raw material is melted on from above.
In the case of the aforementioned methods, the energy for heating up and melting on the silicon raw material is on the one hand introduced via heat conduction aid heat radiation firstly into the melting crucible, in order then to be forwarded via heat conduction and radiation to the material to be melted. On the other hand, the material to be melted is heated on the upper side mainly via heat radiation directly from the heaters. Heat is conveyed inside the melting crucible filled with the material to be melted also via heat conduction and heat radiation. In this case, the material properties, thermal conductivity and extinction play an important part. In addition, the heat conveyance properties are determined by the physical properties of the raw material, as the conduction of heat is impeded at the interfaces.
In order to operate as cost-effectively and energy-efficiently as possible, it is desirable to make the volume of the melting crucible as large as possible, in order also to obtain correspondingly large silicon crystals. The large crucible volume is accompanied by a longer melting-on time, as the amount of heat introduced into the crucible is limited by the size of the surface, which is effective for the absorption of heat, of the material to be melted. A further limitation results from the limitation of the crucible temperature, as the crucible materials do not withstand elevated temperatures and the sensitive material to be melted does not survive undamaged intensive overheating above the melting point without contact reaction with the crucible.
Absorbent materials can be heated via the introduction of electromagnetic alternating fields. In this case, suitable selection of the frequency allows a penetration depth which is adapted to the crucible dimensions to be selected, so that the material to be melted can be heated also in the volume. However, in the case of high temperature dependence and at a relatively high height of the melting crucible, electromagnetic heating is limited to regions close to the surface.
In order to allow more rapid melting-on of the raw material, the preheating of raw material during recharging into the melting crucible is known from the prior art.
DE 32 17 414 C1 discloses the preheating of cullet during recharging into a melting vat of a glass smelting plant. Used for this purpose is a plate heat exchanger, in the intervals in which cullet is constantly replenished. During operation the same amount of cutlet is supplied to the intervals and removed at the lower end thereof by a shaker (vibrating) conveyor. The waste gases which accumulate during the melting process are passed through the plate heat exchanger at a temperature of approximately 420° C., as a result of which the cullet is preheated to a temperature of approx. 245° C. Vertical movability of the plate heat exchanger prevents caking-on of the cullet and also bridging in the interspaces of the plate heat exchanger. However, the construction is comparatively complex.
DE 42 13 481 C1, which corresponds to U.S. Pat. No. 5,526,580 and U.S. Pat. No. 5,412,882, discloses a corresponding plate heat exchanger, wherein a drying step precedes the preheating of the cullet. For this purpose, the moisture in the material to be melted is evaporated in a dry zone in the feed region of the material to be melted by way of separate supply of hot heating gas in hot gas flows which have already cooled down.
Corresponding preheating by way of heat exchanger tubes is known from U.S. Pat. No. 4,353,726, also for the recharging of powdered materials in the manufacture of glass.
JP 07-277874 A and English Abstract thereof disclose the recharging of liquid silicon in the manufacture of monocrystalline silicon using the Czochralski method. For this purpose, a silicon raw material rod is melted on directly above the melting crucible with the aid of a melting heater. The melted-on silicon flows directly and continuously into the melting crucible.
JP 2006-188376 A discloses the production of a monocrystalline material using the Czochralski method, wherein polycrystalline raw material is recharged as a result of the fact that a rod-like polycrystalline raw material is melted on. For this purpose, the rod-like raw material is held in a holding body and immersed into a raw material melt in the melting crucible.
JP 07-118089 A discloses a method for producing a silicon monocrystal using the Czochralski method, wherein granular polycrystalline raw material is introduced into the melting crystal via a supply pipe. In order to prevent SiO formation during recharging, a reducing gas (i.e. hydrogen or a hydrogen/insert gas mixture) is blown onto the surface of the silicon melt.